This invention relates to load cells used in electronic scales and other weighing apparatus and, more particularly, to such load cells having improved moisture-proofing means.
A cantilever-type load cell which is commonly used in electronic scales generally comprises a load-sensitive element having an upper beam and a lower beam forming a pair. Two strain-generating parts are formed on each of these two beams and strain-detecting elements such as strain gauges are attached individually to such strain-generating parts on the beams so as to measure the tensile and compressive strains at these parts caused by a load applied to the load-sensitive element and to thereby determine the magnitude of the load itself.
Moisture-proofing means are sometimes provided to these strain-detecting elements of a load cell of this type in order to protect these elements against moisture and humidity and to thereby enhance their durability. Examples of moisture-proofing method which has been tried include covering the elements by a material such as silicone rubber and butyl rubber. Such organic polymer materials, however, are moisture-permeable to a certain extent because of their molecular structures. Although they do not totally lack in moisture-proofing effect, they are not sufficiently effective especially when the load cell is used in a very humid or moist environment.
In view of the above, there have been attempts to cover the strain-detecting elements with aluminum foil or a metallic moisture-proof sheet or cover of a layered structure with a synthetic resin film on aluminum foil. Since such sheets and covers are not sufficiently elastic, however, they tend to impede the free deformation of the load-sensitive element corresponding to the load which is applied thereon. Thus, it is now being attempted not to attach these sheets and covers directly on the strain-detecting elements but to provide a space therebetween. Japanese Utility Model Publication Jikkai 61-30838, for example, discloses a load-sensitive element with a pair of upper and lower beams each characterized as having an indented part on its surface such that load-sensitive elements are placed in these indented parts which are then sealed by cover means. Japanese Utility Model Publication Jikkai 59-183639, on the other hand, discloses a method of providing water-proofing putty on such beams above and below the load-sensitive element around the positions where strain-detecting elements are to be attached such that water-proofing sheets placed thereover can tightly seal the strain-detecting elements with a space provided therearound.
The prior art attempts described above are not satisfactory for several reasons. Firstly, it requires additional production processes to provide such moisture-proofing means on the surfaces of both upper and lower beams for the load-sensitive element.
This implies an increase in the production cost. Secondly, there still remain problems relating to the accuracy of measurement as well as moisture-proofing because, even if metallic moisture-proofing sheets and covers are attached indirectly by providing a space around each strain-detecting element, it is impossible to completely prevent them from affecting the strain of the load-sensitive element according to the applied load. If these sheets or covers are attached to the beams, their effects may be magnified and adversely affect the accuracy of measurement. Since moisture-proofing must be effected at two positions, furthermore, there is an increased probability of occurrence of a defect which may eventually affect the durability of the load cell.